Help needed with understanding accuracy in measuring tools. I want to go straight :-)

[Boyan Silyavski]

Hi, i need some help there.

I am on my way to purchase some precision instruments needed for my build and hopefully for next builds. On the cheap

Basically i need instruments for precision squaring of the CNC.

1. I just bought from ebay Super accurate pair 3" and 3.5" machinist square accurate to +/- 20 sec of arc

Sellers description:

This is a pair of solid machinist squares. One square is 3" x 3" with a 90 and dual 45 degree angles and the other is 3 1/5" x 2" with 90, 60 and 30 degrees angles. Angular accuracy is insanely accurate to 20 seconds of arc. So just what is a "second of arc?" A full circle consists of 360 degrees. One degree can be divided into 60 minutes of arc. Each minute of arc contains 60 seconds of arc, so a second of arc is an angle that is 1/3,600 of a degree. 20 seconds of arc would be 1/180 of a degree, or a deviation of 0.000055" per inch. These squares are made from hardened carbon tool steel and are ground and lapped for straightness and parallism. These are the most accurate squares you will find for this price anywhere.

2. Now i am contemplating of buying from aliexpress 2 pieces of Universal Bevel Protractor Angular Dial Length 150mm 320 Degree

The first buy will be used to adjust the 2 universal protractors squarish.

3. Now i need a straight edge. It will be used to check rails mounting surfaces and so on. Aslo i intend to use it for connecting it to the 2 squarish protractors and check and adjust rails parallelism. Here comes the question.

I will like this to be as large as possible without breaking the bank. I saw on ebay 2 options:
a/Igaging 36" straight edge beveled precision ruler hardened steel

Sellers description:
These straight edges have hardened, satin chrome finished blades that are ground and lapped for straightness and pallalelism and have true right angle on all edges with one long beveled edge. Beveled edge is graduated in 64ths and is precision photo etched and then black filled for easy readability. Rules are 36 3/16" long, 1 9/16" wide and 3/16th thick and weigh in at almost 4 pounds. These are substantial rules. Accuracy is guaranteed to be +/- 0.0015" (one and a half thousandths of and inch) over the length of the 36" rule. Rules come in an imitation leather pouch.These straight edges are perfect for setting up machines, checking work for straightness and scribing straight lines. These rules are US engineered and manufactured to strict tolerances in factories overseas. These are well made and precise and I am convinced they offer a reasonable alternative for anybody unwilling to spend the money required for the more premium line of US made precision tools.

b/ Maun Carbon Steel Straight Edge 48in

sellers description:
Accuracy is 0.01% of the length of the straight edge.

Size.48 inch.
Width 2in.


Hence the questions:

- am i on the right way?
- obviously the maun straight edge is longer 42 inch versus 36 inch, but what does the stated accuracy mean? I can not understand it, percentages, etc? can somebody translate?
-any other options, better or cheaper? And preferably from Europe.
-other ideas?

Thanks!

[Clive S]

The way I see it is that the straight edge will not vary by more than about .12mm over its length. ..Clive

[EddyCurrent]

There's nothing wrong with getting more gear but do you need it ? what's wrong with measuring diagonals with a strip of wood having a chisel point at each end to get into the corners ? I'm thinking those squares are quite small so that over a distance of say 1000mm any error would be magnified. There again I've just rechecked the prices and you might as well get them.
There's a 'law' that goes something like this, in your workshop you need 3 things, tools, materials, yourself, but there's always just enough room for 2 of those.

[Boyan Silyavski]

I was interested in the straight edge, for checking the gantry profile after welding. And as i have nowhere to machine it and havent found around where to do it, i was contemplating checking and filing until is flat. i believe its called " elbow grease " in English.

My idea also was to do the same with Bosch profile i have, as its quite straight. The small triangles i bought as i needed sth really straight as a starting point for all projects.
maybe i am wrong, but how to make a precision machine if i don't have a straight ruler to draw straight line?

Apart from parallel i would like the Y rails / the long ones/ to be straight, not just following each other curvature. Any other way to do this with simple tools like a Bosch profile?


So you say the price is ok?

[EddyCurrent]

I think they look very nice for the price and I agree a straight edge is essential but I'm not sure you really need the other items though I'm sure they will be useful for some jobs.

As Clive said 0.01% of length = (0.01/100)x1200mm = 0.12mm

[Jonathan]

It all depends on how accurate you want to be. I'm sure a lot of people make a machine which worsk, without using this equipment, but they wont make a really accurate machine without it. For the machine I recently made I used a 24x12" surface plate to measure and correct the rail straightness. Fortunately I won it for about £20 at an auction!

Apart from parallel i would like the Y rails / the long ones/ to be straight, not just following each other curvature. Any other way to do this with simple tools like a Bosch profile?

Not cheaply. People have experimented on CNCzone with using a tensioned wire to make a straight(ish) line for reference, but it's not going to be as good as the proper device.

[Boyan Silyavski]

Thanks guys,
so it seems clear to me that i will pull the trigger at the straight edge. A sort of investment.

The 2 angle finders from aliexpress will wait for the moment. My idea was to use them like this:

Look at the gantry picture bellow. Using the straight edge or epoxy , like for the long rails, i could flatten the upper side/1/
But then how to flatten the opposite side? I don't have a fixed gravity epoxy formed horizontal surface at home. If it was my proper house, i would have done it long ago. So just flipping sides and pouring epoxy would be simple. So i thought of connecting the 2 angle finders un U like shape so i can measure distance and parallelism bots sides. So using the epoxy leveled top or bottom as a reference i could file , sand and so on, the other side. if they were on one plane , it would be easier, but damned overhang...i don't like it.

Anyways, i bought 2 packs of 1kg west system epoxy kit 105/209. After pouring some where necessary, the rest i will use to form a flat table surface and fix some short adjusting legs. As i have a very precise inclinometer / ~0.5mm per 3m, or so i calculated it when i bought it, don't remember but was the most precise on ebay/ i can fix me a portable level base precision surface for similar projects. I need a bigger garage:-)

Another thing is that just now i found how to make precision straight edge at home. 3 edges in fact. very interesting read. Now i am contemplating how to make it in reality. will need some polishing paste. maybe vibrate the edge for quicker removal. or slowly, like making a katana, while watching movies at night at home...

PS. i want to be as most as precise as possible, cause i know that even if you try, you are not. So if you don't try, its even worse